Potential Lifshitz transition at optimal substitution in nematic pnictide Ba 1- x Sr x Ni 2 As 2 .

BaNi ₂ As ₂ is a structural analog of the pnictide superconductor BaFe ₂ As ₂ , which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature ( T _c ). To understand the mechanisms responsible for enhancement of T _c , we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba _{1- x} Sr _x Ni ₂ As ₂ series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where T _c is maximum. These nested pockets host Ni d _xz /d _yz orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced T _c .